1. Field of the Invention
This invention relates generally to a portable radio telephone system. More particularly, the present invention relates to a private radio telephone system, for use in a local area, such as in an office building or in a factory and of the type wherein, when the portable telephone set is taken out from the area, it can communicate with subscribers of a public telephone network through a public cellular radio telephone system which has already been placed in service in certain urban areas.
2. Description of the Prior Art
A public cellular ratio telephone system has been known in the past as a urban cellular radio telephone system. When its portable telephone set is brought into an office building orina factory and is used to communicate with a subscriber of the private branch exchange for communication, not only the quality of telephone voice drops due to radio wave propagation attenuation by the office building, but also the telephone charge of the urban cellular telephone system must be paid.
As one example of urban cellular ratio telephone systems which improve the radio wave propagation attenuation of this kind, reference is made to the Proceedings of IEEE-VT Conference, 1984, pp 162.
In the portable telephone system described in the publication described above, introduction of a private branch exchange which has radio telephone subscriber circuits is not taken into consideration, so that direct communication cannot be made from the portable telephone set to the subscribers of the private branch telephone system.
On the other hand, if we introduce private branch radio telephone systems that have been commonly considered, the portable telephone set becomes expensive because it must have a radio frequency circuit of the private radio frequency channels in addition to the radio frequency of the urban cellular radio telephone.